This application is a divisional of application Ser. No. 10/943,838, filed Sep. 20, 2004 now U.S. Pat. No. 7,171,133.
1. Field of the Invention
The invention relates to an image forming apparatus of, for example, an electrophotographic type, an electrostatic recording type or the like, and particularly to an image forming apparatus using density detecting means for detecting the density or the amount of adherence of a toner image.
2. Related Background Art
A toner density sensor is shown in FIG. 8 of the accompanying drawings. The sensor comprises a sensor case 29, a light emitting element (LED) 50 and a light receiving element (PD) 51. The detection of toner density is effected by turning on the LED 50 to thereby apply light to a reference toner patch (hereinafter referred to as the patch) on a photosensitive drum 17 which is an image bearing member, and detecting light reflected from the patch or the surface of the photosensitive drum by a photodiode 51. (See, for example, “Electrophotography-Bases and Applications” compiled by the Society of Electrophotography of Japan, CORONA PUBLISHING Co., LTD., Jun. 15, 1988, pp. 286-287.) As the wavelength of the LED 50, use is made of an infrared area, and here, use is made of a wavelength of 950 nm. The relation between the detected reflected light and the toner density exhibits such a characteristic as shown in FIG. 7 of the accompanying drawings and therefore, the density is calculated by the use of this relation. Particularly, a black toner and color toners (yellow, magenta and cyan) differ in the light reflecting and absorbing characteristics of the toner from one another. The black (Bk) toner uses carbon black and therefore absorbs light in the entire wavelength area and therefore, the quantity of reflected light lowers as the toner density rises. On the other hand, the color toners, as shown in FIG. 9 of the accompanying drawings, differ in characteristic in a visible area (400 nm 700 nm) from one another. In the infrared area, however, any toner exhibits a reflecting characteristic and therefore, by using the LED 50 of the infrared wavelength, it is possible to detect any change in the toner density. In the case of the color toners, infrared reflection is used and therefore, the quantity of reflected light increases as the toner density rises.
The toner patch can be formed by forming a latent image on the charged photosensitive drum by exposing means such as a laser, and developing the latent image by a toner by the use of developing means.
The toner patch is formed in a gradation in some cases, and is formed in a plurality of gradations in some cases.
Now, the toner density sensor often has its sensor detecting surface stained with dust or the like including a scattered toner present in an image forming apparatus. In order to prevent the stains, a shutter can be attached to the sensor detecting surface, or in order to remove the adhering stains, cleaning means can be provided, but this leads to the problem of a cost or a space in the apparatus. Therefore, light is applied to the surface of the photosensitive drum to which the toner does not adhere, and the quantity of reflected light therefrom is detected to thereby detect the stain of the sensor surface, and in conformity therewith, the quantity of light of the LED 50 or the output of the photodiode 51 is corrected (see, for example, Japanese Patent Application Laid-Open No. H07-36230).
Also, the output is varied by the eccentricity of the photosensitive drum and therefore, heretofore, phase detecting means has been provided on the image bearing member (see, for example, Japanese Patent Application Laid-Open No. H07-36231), or during image forming, a marker for position detection has been formed as an image and on the basis thereof, sensor output correction has been effected (see, for example, Japanese Patent Application Laid-Open No. H11-295941).
Further, in order to prevent the vibration of a transfer belt or a conveying belt, there is also means for attaching a supporting member to the back side of the belt to thereby stabilize the output (see, for example, Japanese Patent Application Laid-Open No. H06-3886).
The toner density sensor of the above-described construction, however, operates well, but suffers from the following problems.
When correction is effected on the surface of the photosensitive drum, the corrected value deviates greatly due to the eccentricity component of the drum in some cases. Therefore, it is also conceivable to provide, for example, a sensor for phase control and combine such means as will put a detecting position in order (for example, a combination with Japanese Patent Application Laid-Open No. H07-36231), but this requires much cost and suffers from the problem of space. This is also a method of forming a marker (for example, Japanese Patent Application Laid-Open No. H11-295941), but this suffers from the problem that the marker forming time and sequence become complicated. Further, when this method is adopted in a transfer belt, the addition or the like of a supporting member is necessary, and this also leads to the problem of increased cost.